1. Field of the Invention
The present invention relates generally to optical waveguides, and more specifically to active microstructures, such as photonic band-gap optical waveguides, for use in applications such as fiber lasers and amplifiers.
2. Technical Background
Optical fibers formed completely from glass materials have been in commercial use for more than two decades. Although such optical fibers have represented a leap forward in the field of telecommunications, work on alternative optical fiber designs continues. One promising type of alternative optical fiber is a microstructure optical fiber, which includes holes or voids running longitudinally along the fiber axis and is sometimes called a “holey” fiber. The holes generally contain air or an inert gas, but may also contain other materials or vacuum.
Microstructure optical fibers may be designed to have a wide variety of properties, and may be used in a wide variety of applications. For example, microstructure optical fibers having a solid glass core and a plurality of holes disposed in the cladding region around the core have been constructed. The arrangement, spacing or pitch, and sizes of the holes may be designed to yield microstructure optical fibers with dispersions ranging anywhere from large negative values to large positive values. Such fibers may be useful, for example, in dispersion compensation. Solid-core microstructure optical fibers may also be designed to be single mode over a wide range of wavelengths. Solid-core microstructure optical fibers generally guide light by a total internal reflection mechanism; the low index of the holes can be thought of as lowering the effective index of the cladding region in which they are disposed.
One especially interesting type of microstructure optical fiber is the photonic band-gap fiber or crystal. Photonic band-gap fibers guide light by a mechanism that is fundamentally different from the total internal reflection (TIR) mechanism. Photonic band-gap fibers have a photonic band-gap structure formed in the cladding of the fiber. The photonic band-gap structure may be, for example, a periodic array of holes having a spacing on the order of the wavelength of light. The photonic band-gap structure has a range of frequencies and propagation constants, known as the band-gap, for which light is forbidden from propagating in the photonic band-gap structure. To form an optical waveguide (or more generally, a structure that guides electromagnetic (EM) energy), a defect is formed in the photonic band-gap crystal or fiber. The core of the fiber is thus formed by the defect in the photonic band-gap structure cladding. For example, the defect may be a hole of a substantially different size and/or shape than the holes of the photonic band-gap structure. Alternatively, the defect may be a solid structure embedded within the photonic band-gap structure. Light introduced into the core will have a propagation constant determined by the frequency of the light and the structure of the core. Light propagating in the core of the fiber having a frequency and propagation constant within the band-gap of the photonic band-gap structure will not propagate in the photonic band-gap cladding, and will therefore be confined to the core. A photonic band-gap fiber may have a core that is formed from a hole larger than those of the surrounding photonic band-gap structure; in such a hollow-core fiber, the light may be guided within the core hole. The defect is a discontinuity in the lattice structure and can be a change in pitch of the lattice, the replacement of a portion of the lattice by a material of different refractive index, or the removal of a portion of the photonic band-gap crystal material. The shape and size of the defect is selected to produce or support a mode of light propagation having a wavelength that is within the band-gap of the photonic crystal. The walls of the defect are thus made of a material, a photonic band-gap crystal, which will not propagate the mode produced by the defect. In analogy with the total internal reflection optical waveguide, the defect acts as the waveguide core and the photonic band-gap crystal acts as the cladding. However, the mechanism of the waveguide allows the core to have a very low refractive index thus realizing the benefits of low attenuation and low non-linearity.
There has been significant interest in the potential of photonic band-gap guidance in optical fibers. While the theory of guidance in these fibers has been described, actual fabrication and demonstration of optical properties of photonic band-gap fibers has been relatively rare. The photonic band-gap fibers that have been demonstrated have suffered from high loss (or high attenuation); the lowest losses reported have been on the order of 10 dB/km. In order to be of significant practical interest as transport fibers for telecommunications, photonic band-gap fibers must have much lower losses.
Fiber lasers represent a highly efficient means of converting low-coherence pump light into coherent signal light. Fiber lasers have excellent surface-area-to-volume ratio for cooling, are typically flexible for convenient deployment, and are lightweight and inexpensive. These attributes make fiber lasers extremely attractive for a number of applications.
Scaling fiber lasers to higher powers involves increased pumping levels and interaction lengths. However, nonlinear optical effects and surface damage eventually limit the ability to scale to higher powers.
Previously, fiber profile designs with increased effective areas have been used in an attempt to reduce the nonlinear optical effects. The maximum effective area, however, is typically limited by bend loss; larger effective area fibers usually show increased bend loss. Anti-reflection coatings and polished fiber end faces have been used to reduce the surface damage at the fiber-air interface. These attempts allow for increased operational power, but are still limited to approximately 1000 W of average power. Scaling to higher power requires a mechanism similar to double-clad configurations of conventional fiber lasers in order to efficiently convert multimode pump energy into single-mode fiber-laser energy.
Because of the low non-linearity benefits provided by a photonic band-gap crystal waveguide, there is a need to identify fiber profile design structures that produce modes that will enable efficient conversion of pump energy into single-mode fiber-laser energy.
The uses of the photonic band-gap crystal waveguide include those that involve the delivery of high electromagnetic power levels such as in devices for excising material or welding material.
There is a also a particular need to incorporate the low non-linearity of photonic band-gap crystal in a waveguide, such as a fiber, to scale fiber laser operating powers beyond current designs, which can be limited by nonlinear interactions. One example of nonlinear interaction is Stimulated Brillouin scattering or SBS. SBS is a nonlinear optical process that occurs between an optical field and a material density wave. The optical field and density waves in the material interact through the known process of electrostriction. The coefficient describing the strength of this interaction is described by the electrostrictive constant. In standard solid-core optical fibers an incident field can be reflected from the fiber and frequency shifted as a result of the SBS effect. In many applications, such as fiber lasers and fiber amplifiers, SBS can be a detrimental effect. A number of approaches have been developed to circumvent the influence of the SBS process; most involve reducing the intensity through increased effective area or spectrally broadening the optical fields.
The threshold power for the SBS process depends on the spectral width of the pump wave and the effective area of the optical field. In continuous-wave pump fields it can be as low as 1 mW. Thus, the amplification or generation of narrowband continuous-wave optical radiation is difficult to obtain in optical fiber amplifiers or optical fiber lasers due to the limiting effects of SBS.
In hollow-core photonic band-gap fiber (PBGF) the optical field is guided in a void of the fiber cross section. This void could be filled with air, some other gas, a liquid, or could be evacuated to support a vacuum region. Since the electrostrictive constant describing the SBS interaction is proportional to the number density of the material and the number density of gases is about three orders of magnitude smaller than that of glass, the hollow core of a photonic band-gap fiber will have a nonlinear response about three orders of magnitude smaller than a solid-core glass optical fiber.
In addition, the amplification and generation of pulsed optical fields is limited by the nonlinear processes that take place in the amplifier or laser. Since the nonlinear coefficients that describe the various nonlinear processes are proportional to the number density of the core material in an optical fiber amplifier or oscillator, a hollow-core PBGF is ideally suited to suppress these nonlinear effects.
Therefore, there is a need for an optically active hollow-core PBGF to enable various uses, such as amplifying and generating narrowband optical fields or pulsed optical fields with greatly reduced nonlinear impairment.